ABSTRACT Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections (ALRI) in children and high-risk adults worldwide. Our data demonstrate that RSV infection results in a ?leaky airway? by disrupting epithelial apical junctional complexes (AJC), which regulate the airway epithelial barrier. We show that RSV- mediated disruption of AJC is accompanied by disassembly of the perijunctional actin cytoskeleton, and downregulation of cortactin - a key actin-binding protein. Cortactin deficiency has been previously implicated in destabilizing the vascular endothelial and intestinal epithelial barrier. However the causal link between RSV- induced leaky barrier, actin cytoskeletal rearrangements and cortactin deficiency have not been established. In addition, epidemiological reports suggest a strong association between exposure to ambient particulate matter (PM) and increased risk of ALRI. Nanoparticles (NPs) are extremely small PM, with the greater ability to become deposited in distal airways and evade host defenses compared to smaller particles. Our preliminary data demonstrate that pre-exposure of bronchial epithelial cells to NP not only enhances RSV-induced AJC disassembly and actin cytoskeleton disruption, but augments viral infection. Based on our novel observations, we formulated the central hypotheses that a) RSV induces disruption of the airway epithelial barrier by triggering depolymerization of the perijunctional actin cytoskeleton, and by downregulating cortactin; and b) that disruption of the epithelial barrier by nanoparticles worsens RSV-induced airway epithelium injury. We will test our hypotheses through the following Specific Aims: Aim 1: To determine the role of cortactin-dependent actin filament dynamics in RSV-induced airway epithelial barrier dysfunction. Using human bronchial epithelial cells isolated from pediatric donors, and a mouse model of cortactin null mice, we will investigate (i) RSV effect on actin cytoskeletal dynamics, (ii) the functional role of cortactin on actin dynamics and AJC structure, and (iii) the functional roles of Rap-1. We will also use a 3-D human lung organoids model detailing the effects of AJC disruption upon RSV infection, which offers an innovative platform to study complex host-environmental interactions. Aim 2: To determine if nanoparticles enhance RSV-induced disruption of the airway epithelial barrier. Using in vitro and in vivo models, we will (i) characterize the effects of particle size on barrier integrity, (ii) study role of oxidative stress on AJC function, and (iii) define the effects of exposure to NP on AJC dysfunction. The proposed research is significant and relevant to the NIH?s mission as we aim to explore the clinically relevant consequences of RSV infection on airway barrier integrity, and how exposure to environmental pollutants worsens RSV infection. Our approach is innovative because it will provide new mechanistic insight in to the roles of RSV in AJC disassembly, as well as novel insight in to the effects of NPs in enhancing RSV- induced AJC disruption. The identified pathways will provide new targets for therapeutic intervention and the potential for positively impacting the management of RSV disease.